Fuel cells utilize the chemical reaction of a fuel with oxygen to form water in order to generate electric energy. For this purpose, the core component of fuel cells is the so-called membrane electrode assembly (MEA), which consists of an ion-conductive (usually proton-conductive) membrane and an electrode (anode and cathode) arranged on each side of the membrane. Moreover, it is also possible to arrange gas diffusion layers (GDL) on both sides of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. As a rule, the fuel cell is formed by a plurality of stacked MEAs whose electric outputs are cumulative. As a rule, there are bipolar plates (also called flow field plates) arranged between the individual membrane electrode assemblies and they ensure that the individual cells are supplied with the operating media, in other words, the reactants, in addition to which they also serve for cooling purposes. Moreover, the bipolar plates establish an electrically conductive contact with the membrane electrode assemblies.
During the operation of a polymer electrolyte membrane (PEM) fuel cell, the fuel, especially hydrogen H2 or a gas mixture containing hydrogen, is fed to the anode via a flow field of the bipolar plate that is open on the anode side, where an electrochemical oxidation of H2 to form H+ takes place while electrons are released. A (hydrous or anhydrous) transport of the protons H+ from the anode space into the cathode space takes place via the membrane which separates the reaction spaces and electrically insulates them from each other in a gas-tight manner. The electrons provided on the anode are fed to the cathode via an external electric circuit. Oxygen or a gas mixture containing oxygen (for instance, air) is fed to the cathode via a flow field of the bipolar plate that is open on the cathode side so that a reduction of O2 to form O2− takes place while electrons are picked up. At the same time, the oxygen anions react in the cathode space with the protons that have been transported via the membrane, a process in which water is formed. The electric potential generated by the chemical reactions can be tapped via the external electric circuit in order to supply an electric consumer or to charge a battery.
In order to supply a fuel cell stack with its operating media, in other words, the reactants, the fuel cell stack has, on the one hand, an anode supply system and, on the other hand, a cathode supply system. The anode supply system comprises an anode supply path for feeding an anode operating gas into the anode spaces, and an anode exhaust gas path for discharging an anode exhaust gas out of the anode spaces. By the same token, the cathode supply system comprises a cathode supply path for feeding a cathode operating gas into the cathode spaces, and a cathode exhaust gas path for discharging a cathode exhaust gas out of the cathode spaces of the fuel cell stack.
The operation of a fuel cell stack requires a number of peripheral components (auxiliary aggregates). Examples of these include air compressors, recirculation fans, cooling water pumps, valves, sensors, etc. The power consumption of these components is referred to as parasitic consumption since, even though this energy has to be provided by the fuel cell stack, it is not available for external consumers. Since the power available to external consumers is diminished by the parasitic current, the total efficiency of the fuel cell system ηSys is always below the efficiency of the fuel cell stack ηFC (see FIG. 2). Here, in case of a low load (corresponding to a low power withdrawal), the efficiency of the system ηSys drops disproportionally due to the peripheral components that are being operated.
During the operation of the fuel cell stack, it is necessary to avoid high cell voltages and not to fall below appertaining minimum limits for the withdrawn power or load since this can lead to a degradation of the catalytic material of the cathode and anode of the cells. For this reason, as a rule, the fuel cell stack is only operated to an output minimum (operating point C in FIG. 2) and this is then maintained as the idling state. If power requirements are even lower or absent, it is desirable to temporarily switch off the fuel cell stack. This is why fuel cells are controlled in a so-called start-stop mode of operation in order to temporarily put them in a standby mode. The change-over from normal operation—in which the stack is operated with the cathode and anode operating gases while load or power is being withdrawn—to the standby mode (also referred to as the sleep mode) is the subject matter of the present application.
In today's systems, as rule, the change-over to the standby mode is done by switching off the compressor in order to interrupt the feed of cathode gas. Subsequently, the reactants contained in the fuel cell stack finish reacting, as a result of which electric energy is generated that is then withdrawn from the fuel cell stack as discharge current as a function of the voltage until the chemical reactions come to an end. Once a cell voltage of, for instance, 0.4 V per individual cell has been reached, the discharging procedure is terminated so that the fuel cell stack does not reach a degradation-critical state. In order to prevent an excessive pressure differential between the anode side and the cathode side, which could cause damage to the membrane, modern strategies prescribe that the fuel cell stack is only changed over to the standby mode out of operating points involving low gas pressures, in other words, low load withdrawals.
European patent specification EP 2 564 459 B1 describes a method for operating a fuel cell system in which the efficiency of the fuel cell system is ascertained and the system is changed over to a standby mode when a lower limit value is reached.
German patent application DE 10 2009 001 630 A1 discloses that, if the power requirements are low or absent, the fuel cell system is first operated at the point of the best system efficiency in order to charge the battery, after which it is changed over to the standby state. For this purpose, the air compressor is switched off and a target voltage value for the direct voltage network connected to the fuel cell stack is set to a non-critical value. In the meantime, the anode gas feed remains active in order to diminish the oxygen on the cathode side.
German patent application DE 10 2004 034 071 A1 describes a method for switching off a fuel cell system. In order to do so, first of all, an idling pressure of, for example, 1.6 bar is set at the anode and subsequently the anode feed is interrupted. Through the application of a load that is specified as a function of the desired duration of the switch-off procedure, hydrogen is consumed while power is generated. The cathode pressure is adapted in such a way that a maximum pressure differential between the anode and the cathode does not exceed a critical value of, for instance, 0.2 bar. If a prescribed low voltage is reached in the fuel cell, the load withdrawal is stopped. The compressor for conveying air is not switched off until the anode pressure has reached the ambient pressure.